If there is a merger who's contract do we get?
I assume you are asking regarding M&R by your screen name.
During a merger scenario involving two different unions, there is a threshold of percentage of membership which would trigger a representational election. If I recall correctly, the smaller of the two memberships must have at least 35 % compared to the larger group. So if the TWU had more than 65% of the group (which they do today I believe, but most likely will not at time of any merger, due to outsourcing) there would not be an election and and the TWU contract would govern.
However, remember the IAM could start a card drive hoping for discontent from disenfranchised TWU members and if they obtain enough signed cards, they could call for an election ( but this would take some time ).
Now, if the M&R group, and AA fleet take huge hits during BK due to outsourcing (which it kind of looks like they may) their numbers advantage may fall below the 65% threshold and a representational election would be held.
I would expect both unions to attempt to negotiate Transitional Agreements for their respective groups until representational issues can be decided.
The unions could also negotiate who ends up with what areas without an election but I doubt that would happen here as the stakes are very high for both unions... a lot of "dues producers" to be had for someone.
A warning about representational elections... if neither union gets 50% + one vote (if a lot of people choose not to vote -- example -- the TWU gets 47% and the IAM gets 46% and 7% stayed home) you have no union representation, so be sure to vote for someone!
Hope this isn't too confusing, and if I'm inaccurate someone more knowledgeable will surely clear it up.
Good luck.